In recent years efficient automatic systems have been developed for welding girth joints in pipelines. One successful system involves a combination of operations, starting with an inside pass or "stringer bead" operation, where the pipe ends are held in mutual alignment by an internal clamp while an internal orbiting welder travels around inside the joint, as described and claimed in U.S. Pat. No. 3,564,264 and others. For most joints, several sequential welding steps or passes are required. This internal bead then serves not only to hold the pipe pieces together while unclamping and other operations are performed but serves also as a backing for the later passes, performed from the outside, to build the weld from the inside out. Means and methods for completing the operations from the outside are described in such patents as U.S. Pat. Nos. 3,718,798; 3,806,694 and others. A self-propelled carriage, guided in an orbital path around the joint, transports the welder which sets up and maintains an electric arc and operates to lay down molten metal and to fuse it into previous passes, in sequence, until the gap between the pipe ends is filled and capped. Normally, each pass is performed at a separate station, sequentially along the pipeline.
In cases where the pipes to be joined are of thin wall construction, say, below about 0.250 inches, the weld may be completed in only one or two outside passes, added to the internal stringer bead. In many cases the pipe wall is thicker than this and additional passes are required since there is a practical limit to the amount of molten weld metal that can be added and the amount of heat that can be put into the weld by the electric arc at a single pass, while still obtaining a weld of good quality. In any case, it is desirable to complete the welded joint in as few separate operations and stations as possible. As successive passes or steps are usually performed at separate and consecutive stations along the pipeline, each station must have a complete welder unit, i.e., at least a guide track, a carriage, and a welder device transported by the carriage. One or more operators are required at each station; hence, economy of manpower as well as of equipment dictates that the number of stations be held to a minimum, consistent with obtaining high quality in the weld at every step. Because of the hazards of leaks or breaks in the line, and the waste of valuable products which might occur if welds should fail, strict standards are required in pipeline construction. The industry, therefore, is interested in welding processes which will produce high joint quality. The costs of manual operations are very high; hence, there is a demand for a simple, automatic welding means and method that can complete work of high quality with a minimum of costly equipment, manpower and work stations.
On land, where space may not be a problem, it may be less important to reduce the number of stations that are needed to complete welding operations, although economy of operation is always important. At sea, or in any offshore situation where the pipeline is built on a barge and is lowered step by step to the ocean floor or other underwater surface as it is completed, the number of stations permissible becomes highly critical. To insure against possible pipeline breaks with consequent damage to the environment, and in view of the high costs of repair of underwater lines, as well as the value of oil, gas, and other products to be transported, the pipe used often has comparatively very thick walls. In many cases five, six or even more separate welding passes may be made to complete a weld in such pipe. By prior art methods, as many stations as passes are required, spaced consecutively along the line by a length of "joint" of pipe, commonly 40 feet per joint. With such work, using six stations, a line over 200 feet long is necessary, and longer if further stations are required. Aside from the welding, stations are usually required for stacking the pipe, bringing it to the line, etc., and, after the welding, for coating the welded joint, encasing it in concrete, etc., as is well known in the art. Space on any barge is limited and it is obviously desirable to complete the welding and all other operations in as few stations as possible. Even on land, where space is not so critical, it is desirable to keep the operation as compact as is convenient, for better communication, utilization of manpower, etc. An object of the present invention is to increase the portion of a welded joint that can be made at a single station. This saves equipment requirements as well as space.
Quite independent of the considerations mentioned above, there are desired physical and/or metallurgical effects on the weld that can be achieved by a dual weld technique. When welding passes are spaced apart by long time intervals, heat input must be higher to compensate for greater cooling, which can adversely affect the physical properties of the weld. Depending on the energy input of the arc and the heat absorbing and conducting properties of the metal, the cooling rates for the joint may be excessive. Among other things, high cooling rates may cause undesirable brittleness or high hardness and stresses in the welded joint. By making at least some in a series of plural passes close together, time-wise, some of these effects are avoided. In some cases, as in the prior art where single and separate welding passes are used, a special heat treatment may be required after the welded joint is completed, to normalize and/or to relieve stresses. Better heating and cooling rates may be obtained by multiple weld passes in close succession to eliminate the heat treating step and the extra station it requires. If six passes, for example, can be performed at three or four stations, while also eliminating the extra heat treating station, three or more stations can be dispensed with, greatly facilitating the operation, and reducing costs, especially on offshore barge operations.
Hence, another object is to better control heat input and cooling rates by combining or doubling up weld passes on a single apparatus. Preferably, multiple weld passes are performed at most stations, reducing station equipmment, and personnel requirements by as much as one-half. In theory, perhaps, more than two passes may be made with each piece of apparatus, but to obtain the desired metallurgical results, two passes are satisfactory. Experience has shown it to be very difficult for an operator to control more than two simultaneous passes or to restart more than two passes in case of a malfunction, unless conditions are unusually favorable.
It is known, of course, to use multiple arcs in tandem for welding, particularly in straightline operations. Such have been used in welding together the straight adjoining edges of metal plates. To apply this principle to pipeline girth joint welding, however is not so simple for several reasons. In the first place, pipes vary widely in diameter, that is the curvature that must be followed by the device may be greatly different from one job to another. This introduces one set of complications. The apparatus must travel in orbit, another limitation. Also, for deep gaps, as are to be welded in thick wall pipes, the width of fill needed may vary considerably, from bottom to top. There may be need, for this reason, to oscillate or move from side to side the welding head which is filling a wider part of the gap than its predecessor, whereas another head filling a narrower part of the gap should be oscillated at a narrower amplitude or perhaps not at all. To devise a simple apparatus that will accomplish such operations, without undue complications, is another object of this invention.
The success achieved in the past by the welding apparatus described in U.S. patents mentioned above, and others related thereto, has been based to a considerable extent on the fact that the devices are simple, rugged, but still highly precise and rapid in their operations. The devices are capable of fine adjustment so as to follow in the plane of the joint with high accuracy, and to stand off the optimum distance from the work, to oscillate or reciprocate with fine control from side to side of the joint in a path that will give optimum fill with effective side bonding of the joint, etc. To add another head to such an apparatus, thereby essentially doubling its capacity, without loss of high precision, is another and important object of the present invention. In thick wall pipe, it is often desirable to use a tapered gap or kerf, i.e., one that is wider at the top or outside, and an object here is to fill each layer of molten metal completely across the gap. Consecutive layers thus become progressively wider and an aspect of the present invention is the concept of increasing the oscillation amplitude as the consecutive passes are made, without using a separate oscillator.